Nitrate is the most prevalent ground-water contaminant worldwide. Nitrate originates from agricultural, sewage-disposal, and industrial practices from both point and nonpoint sources. Through not exclusive to the subsurface, nitrate contamination is much more pervasive in ground water because nitrate has a relatively long residence time in that environment. Ground water is also the most common drinking water source for both humans and livestock in rural and suburban areas of the United States. Thus, when the nitrate concentration in water from a supply well exceeds drinking water standards (i.e., 10 mg/L nitrogen), the burden typically falls upon the individual user or household to deal with the problem.
The options currently available to treat nitrate contamination on a small scale level are limited. Since nitrate is stable in aqueous solution, it can only be safely removed chemically by techniques such as anion exchange. This can be costly, replaces one salt for another, and at times is ineffective, depending upon the composition of other salts in the water. Moreover, there is the need to dispose of the nitrate that has been removed. Additional, cost-effective technology to remove nitrate from drinking water is needed: technology that is effective, safe, and practical at the household and livestock supply scales.
Processes for eliminating nitrates from water by denitrification in microbiological reactors are known. These processes, such as those conducted in rising current reactors containing a granular denitrifying biomass, have been described, for example, by Lettings et al., (1980) and by Timmermans, (1983).
For waste waters in particular, different reducing agents such as sugars, less expensive biodegradable organic material, including cellulose and ethanol, have been used. However, only ethanol has been used in treating water that is to be potable. These conventional reducing agents have the disadvantage that they dissolve in water and reduce the quality of the potable water produced. Therefore, it requires another step to eliminate these reducing agents before the water is ready for use.
Verstrate et al., in U.S. Pat. No. 4,696,747, describe a process for eliminating nitrates by biological conversion in the presence of hydrogen gas. This process uses alcaligenous eutrophic bacteria, with Pseudomonas denitrificans and Micrococcus denitrificans being the preferred microorganisms. However, these bacteria cannot grow and remain active in a hydrogen-fed bioreactor when nitrate is not present, particularly when oxygen is removed.
Hydrogen-oxidizing bacteria, some of which are capable of denitrifying nitrogen oxides, are well known and have been studied in detail for many years (Aragno & Schlegel, 1981). Pilot-scale industrial plants that use mixed-culture populations of hydrogen-oxidizing denitrifiers have been operated in Belgium (Liessens et al., 1992) and Germany (Gros et al., 1988) to produce drinking water from nitrate-contaminated ground water. These plants are engineered to produce up to 50 m3 per day. They are technically complex, require a commercial supply of hydrogen, and trained experts to ensure an adequate function on a daily basis. As a result, an analogous approach or device has not been developed to treat nitrate on a small-scale basis.